Most cellular communication in use in the world today utilize either the GSM (including UMTS) or CDMA (IS-95 or CDMA2000) communication systems. These systems transmit speech over a voice traffic channel using a modulated carrier wave. For example, 2G GSM uses GMSK modulation and IS-95 CDMA uses PSK modulation. Prior to modulating the speech for wireless transmission, the voice input is run through a speech compression circuit such as a vocoder to compress the voice input into a smaller amount of data. This reduces the amount of speech data that needs to be transmitted via the wireless network, thereby permitting the use of a smaller bit rate and a greater number of users sharing the same communication system.
Transmission of non-speech data via the voice channel is also possible and techniques for doing so are known. These techniques typically use a modem at each end with a predefined connect tone or series of tones being used to alert the receiving modem of a requested data transmission. The various attributes of the data connection can then be negotiated by the two modems. To implement this approach, a tone detector is used to recognize the tone from the received signal. However, the ability of the tone detector to recognize the tone is impacted by the vocoder used, as different speech compression techniques have different effects on the tone as it passed through the vocoder.
The speech codecs used by different vocoders commonly use various forms of linear predictive codings (LPC); for example, 2G GSM uses a RPE-LPC speech codec, while IS-95 CDMA uses a variable rate CELP codec. These predictive compression techniques are designed specifically for voice encoding and, as such, are designed to filter out noise and other non-speech components. As a result, the transmission of connect tones and digital data (such as ASCII text, byte codes, binary files) can be problematic since the vocoder processing can corrupt the tones and digital data, making them unrecoverable at the receiving end of the transmission.